Furnace and method of operating



Jan. 9, 1934. F. H. DANN-:Ls 1,942,587

FURNACE AND METHOD OF OPERATING v Filed Nov. 14, 1950 2 Sheets-Sheet l l .NVENTOR ERED Ha DAM/ELS h ATTORN EY 2 Smets-sheet 2 F. H. DANIELS FURNACE AND METHOD OF OPERATING Filed Nov'. 14, 1950 Jan. 9, 1934.

INVENTOR F555 H2. DAM/ELS ATTRN EY Patented Jan. 9, 1934 UNqITED STATES PATENT OFFICE FURNACE AND lVIETHOD OF OPERATING Application November 14, 1930 serial No. 495,686

10 Claims.

This invention relates to the burning of fuel,

and more particularly to a furnace for burning fuel in pulverized form and to a method of operating such a furnace whereby efficient combustion and other desirable results are obtained.

Great difficulty has been encountered in the past in operating pulverized fuel burning furnaces, because of the fact that the fuel must be burned at different rates to take care of a Varying demand. The furnace may be used to heat a steam boiler, for example, and the demand for steam may be very great at certain times, while at other times it may be very low. If the furnace is made of sufficient size and otherwise properly designed to burn efficiently the necessary amount of fuel to supply the maximum demand, itis often found that when the requirements are reduced to the minimum the furnace temperature will drop too low and it will be impossible to maintain the ignition of the pulverized fuel flame.

It is accordingly one object of the invention to provide a furnace and `method of burning pulverized fuel whereby great flexibility of operation may be obtained and efcient combustion may be made possible over a wide operating range without encountering any diculty with ignition at the low combustionl rates.

Pulverized fuel furnaces are usually so constructed and operated as to deposit the ash from the burning fuel in the form of a ne, dry powder.

This ash has no commercial value and the' problem of its disposal is a serious one. It is not practical to use this material for lling purposes, for it is too easily carried away by the wind. In order to overcome this difficulty it has heretofore been proposed to operate a pulverized fuel furnace at such a high temperature that the ash is melted and collects in the form of a liquid slag at the bottom of the furnace, from which it may be tapped at intervals and solidified by a jet or spray of water. This method introduces new difficulties and new problems, particularly when the fuel used has a high ash fusion temperature. It is found that the maintenance of the necessary high temperature in the furnace causes injury to the furnace walls, and the expense of repairing these walls offsets any advantage to be gained by the use of the so-called slag tap process. Moreover the temperature in any furnace chamberis a function of the rate at which fuel is burned therein. Hence if the demand for heat decreases and the fuel supply is reduced, thel furnace temperature will drop. In some cases only a slight drop will be necessary to bring the furnace temperature below the fusion point of the slag, whereupon the slag will form a solid mass in the furnace and its removal will be very dicult if not impossible without destroying the furnace bottom.

It is accordingly a further object of the invention to overcome these diiculties and to provide a furnace and method of burning pulverized fuel which will produce a readily salable ash without necessitating continuous operation at a high combustion rate and without causing a great expense for maintaining the furnace in a proper operating condition.

It is a further object to provide a furnace construction in which the ash may be kept in a molten condition during the operation of the furnace irrespective of the rate of combustion of the fuel or the demands on the furnace.

In burning pulverized fuel it is found that a considerable quantity of the ash is carried out of the furnace by the hot gases. This material frequently includes an appreciable amount of combustible matter, which of course represents a loss of a part of the heat energy available in the fuel as fired. Moreover, some of this so- 30 called fly ash is usually ejected from the furnace stack and deposited over the surrounding district.

It is accordingly a further object of the invention to provide means for recovering heat .35 energy present in the combustible matter which is carried out of a pulverized fuel furnace by the gaseous currents, and for reducing the incombustible matter to a readily disposable condition.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts and steps of the process set forth in the specification and covered by the claims appended hereto.

In accordance with my invention, I provide a furnace having a primary combustion zone and a supplemental combustion zone. Under ordinary conditions the primary zone serves for the combustionvof the greater part of the pulverized fuel and its fuel supply may be regulated in accordance with the demands on the furnace or otherwise to give the desired rate of heat release. The supplemental combustion zone may serve a manifold purpose. Combustion may be effected therein to supplement the burners of A the primary zone and assist in maintaining ignition therein, as well as to provide a constant source of heat when the main burner is not being utilized. In addition, the supplemental zone may serve to melt the ash received from the primary zone, and for this purpose the supplemental Zone may be maintained at a required temperature, as by means of burners to which pulverized fuel, gas or oil is supplied. In this case, the supplemental zone communicates with and is preferably located beneath the primary zone for reception of the ash particles therefrom. One embodiment of such a furnace may comprise a large :mainA furnace chamber, where the major portion of the pulverized fuel is burned, which communicates through a restricted opening with an auxiliary furnace chamber located therebeneath. This lower chamber is provided with a tap hole through which the melted slag may be discharged as desired. As a further feature of this invention, that portion of the ash which leaves the furnace with the gaseous products of combustion may be separated, as by means of baliies, and returned through a conduit to the auxiliary furnace chamber where the combustible matter is burned and the residue is melted.

Referring to the drawings illustrating one embodiment of the invention and in Which like reference numerals indicate like parts,

Fig. 1 is a sectional elevation of a furnace for pulverized fuel, the section being taken on the line 1--1 of Fig. 2; and

Fig. 2 is a section on the line 2-2 of Fig. 1.

The embodiment illustrated in the drawings comprises a primary combustion Zone shown as a main furnace chamber l0 having a front wall 11, a rear wall 12, and side Walls 14. These walls are shown supported by structural steel girders 15, and are formed of a suitable refractory material, such as re brick. Pulverized fuel is introduced into the furnace, together with the air necessary for combustion, by means of burners 16 mounted on the walls 11 and 12. Each burner 16 may be supplied with pulverized fuel from a unit pulverizer 18, which is connected to the burner by means of a pipe 19. The number and arrangement of these burners will depend upon many considerations, such as the maximum and minimum amounts of fuel to be burned per hour, the characteristics of the particular fuel, etc. In the embodiment illustrated there are four burners, two on each of the furnace walls 11 and 12. l

While a furnace of the type illustrated may be used for various purposes, it is particularly adapted for the generation of steam, and I have therefore shown a steam boiler of the well-known cross drum type mounted above the main furnace chamber 10 to absorb heat from the furnace gases. This boiler comprises a front header 21, a rear header 22, rows of steam generating tubes 23 extending between the headers, and a steam and water drum 24. Bafiies 26 and 27 serve to deect the furnace gases and cause them to pass three times across the tubes 23 before reaching the uptake 28.

The bottom of the furnace chamber 10 in the embodiment illustrated is formed by inwardly sloping wall portions 30 (Fig. 2) which are in reality extensions of the side walls 14. These walls 30 do not meet at the center, but a narrow restricted opening 31 is left between them through which the incombustible residues from vthe main furnace are discharged. The walls 30 preferably slope rather steeply, so that such residues as fall thereon will slide downwardly under the influence of gravity. The lower portions 32 and 33 of the, furnace walls 11 and 12 are likewise preferably arranged to slope steeply toward the opening 3l.

In order to prevent the temperature in the main furnace chamber 10 from becoming excessive and causing injury to the furnace walls when the fuel is being burned at the maximum rate, I preferably provide means in addition to the boiler which will absorb heat from the furnace. In the embodiment illustrated, this means comprises a number of closely spaced water wall tubes 35 which are located adjacent to the inner surfaces of the furnace side walls 14 and extend along the sloping bottom portions 30. These tubes 35 absorb radiant heat at a high rate and serve to generate a considerable amount of steam. The number, size and arrangement of the water wall tubes will depend upon the characteristics of the fuel and the rate at which it is to be burned. It is ordinarily desirable, with all except the poorest fuels, to provide a sufcent amount of water cooled surface exposed to the radiant heat to maintain the furnace temperature at all times below the fusion temperature of the ash, so that the incombustible residues will be deposited in the form of a dry powder. In case the fuel to be burned is of a very low grade, a certain amount of slagging may be unavoidable, but the tubes 35 extending along the sloping bottom surfaces 30 will chill the slag and prevent it from adhering.

The necessary water circulation through the water Wall tubes 35 may be obtained in various ways. In the embodiment illustrated, the lower ends of the tubes are bent outwardly and connected to headers 37 which are supplied with water from the boiler drum 24 by means of downcomer pipes 38. At their upper ends, adjacent to the boiler, the tubes are bent outwardly through the side walls 14 and connected to headers 40. These headers are connected to up-comer pipes 41 which deliver the steam generated in the water wall tubes to the boiler drum 24 through a header 42 and connecting tubes 43.

Directly beneath the discharge opening 31 of the main furnace chamber I provide a supplemental combustion zone, which is illustrated as an auxiliary furnace chamber 45 having a front wall 46, a rear wall 47, side walls 49 and a bottom wall 50. All of these walls may be formed of a suitable refractory material, such asfirebrick, supported by a concrete foundation 51. The upper surface of the bottom wall 50 is preferably protected by a layer of high grade molding sand 52 rammed rmly in place. This auxiliary furnace chamber 45 is preferably much smaller than the main furnace chamber 10, and it is arranged to form an ash pit to receive and collect all the incombustible matter precipitated from the main chamber, under the influence of gravity.

The auxiliary furnace chamber is preferably maintained at a temperature considerably higher than that in the main furnace, or well above the fusion temperature of the ash being deposited therefrom. While this temperature may be obtained in various ways, I prefer to accomplish my purposes by burning asuitable fuel, such as pulverized coal, oil, or gas in suspension in the auxiliary furnace. For this purpose I have shown a suitable burner 54 mounted on the wall 47 and supplied with pulverized fuel through a pipe 55 from a unit pulverizer 56 driven by an electric motor 57. The burner 54 delivers the fuel to the furnace with the air necessary for combustion and produces a hot turbulent ame.

It is desirable that the ame should extend throughout substantially the entire length and width of the auxiliary furnace, Without impingement on the walls. This arrangement will produce a tube reverberatory action, and there being no opportunity for the incombustible material in the bottom to radiate heat upwardly to the boiler, this material will reach nearly the temperature of the flame and will be maintained in the form of a liquid slag. In some cases it may be found difficult to completely ll the furnace with ame from a single burner and to maintain the necessary high temperature. Under these conditions, a second burner 59 similar to the burner 54 may be mounted on the furnace wall 46, and supplied with pulverized fuel through a pipe 60 from a unit pulverizer 6l driven by an electric motor 62. This arrangement will produce a zone of very high temperature at the center of the furnace Where the two pulverized fuel ames meet.

The liquid slag which collects in the chamber 45 may be tapped off at intervals or continuously through a small opening 64 in the wall 46. This opening may be plugged with clay when it is desired to allow the slag to accumulate. The slag may be disposed of in any suitable manner. In the drawings, I have shown a spout 65 below the opening 64 which serves to deliver the slag into the path of a water jet 66 from agpipe 67. The water jet chills and solidies the slag into a granulated form which is satisfactory for rilling purposes and hence readily salable. An inspection door 68 in the wall 46 permits the operator to view the interior of the auxiliary furnace and thus ascertain the character of the flame and the amoint of the slag accumulation.

In order to permit the operation of the auxiliary furnace at a high temperature without causing the expense for maintenance of the furnace walls to be excessive, I preferably provide a series of closely spaced water wall tubes 70 which are located adjacent to the inner surfaces of the side walls 49. Water is circulated through these tubes in any desired manner. In the embodiment illustrated, the upper ends of the tubes are bent outwardly through the walls and connected to headers 7l, while the lower ends are bent outwardly through the walls and connected to headers 72. Water is delivered to the headers 72 by means of pipes 73 leading from the down-comers 38, and the steam generated passes out of headers 7l through pipes 74 leading to the upcomers al.

Near the bottom of the wall 47 I have shown a tight fitting door 76, which it may be desirable to provide under some conditions. This door permits access to the furnace and has other advantages. For example, if the pulverizers 56 and 6l should require repairs, the auxiliary furnace may be used temporarily as an ordinary ash pit in connection with the main furnace chamber, which :may be kept in continuous operation, and the ash may be removed in solid form through the door 76.

.I preferably provide means for recovering from the gases which leave the furnace as large a part of the fly ash as possible and for returning this material to the auxiliary furnace chamber where the combustible matter is burned and the incornbustible matter is melted and mixes with the liquid slag on the furnace bottom.

Any suitable means may be utilized to separate the fly ash from the gases. I have illustrated one simple arrangement in which the baille 26 is shaped to provide a p'cclret 78 between the baffle and the boiler header 22. This pocket is located beneath the last pass of the boiler, and the arrangement is such that the deposit of fly ash in the pocket is aided by both gravity and centrifu-h gal force, since the gases are compelled to take a curved path at high Velocity around the bale 27. A pipe 79 leads downwardly from the pocket 78 to an opening 80 in the rear wall 47 of the auxiliary furnace chamber, and this pipe is provided with a, sliding gate or valve 81 by means of which the delivery` of iiy ash to the furnace chamber 45 may be controlled. This valve will ordinarily be opened only at intervals by the operator. to allow the accumulated fly ash to pass downwardly through the pipe under the influence of gravity.

In the operation of my improved furnace, I preferably supply fuel to the auxiliary furnace chamber 45 at a constant rate, regardless of variations in the demand for steam. In this way I maintain a substantially constant temperature in the auxiliary furnace. The rate of fuel supply is preferably high in proportion to the size of the auxiliary furnace chamber, so that the temperature in this zone is at all times above the fusion point of the incombustible residues deposited from the main furnace chamber, but the amount of fuel burned in the auxiliary furnace chamber is much less than that burned in the main furnace chamber 10 at maximum rating. As the demand for steam is reduced from the maximum, I decrease the rate of fuel supply through the main burners 16, and this rate can be reduced to any desired point without encountering difliculty, since the auxiliary furnace provides a sort of pilot re which maintains the ignition. If the demand for steam becomes still less, the fuel to burners 16 in the primary combustion zone may be entirely cut oil and only the burners 54 and 59 utilized. This method provides extreme flexibility of operation, and yet the ash is always maintained in a molten condition even at the lowest ratings, and can be tapped out at intervals through the opening 64. If it is desired to operate at an even lower rating, the fuel supply to the auxiliary furnace chamber may be reduced. This will of course reduce the temperature in this Zone, and at extremely low ratings the ash will have to be removed in dry form through the door 76.

It will be apparent that the expense for maintenance of my improved furnace will be much less than with prior arrangements. Even at the highest rating the temperature in the main furnace will be below the point where the furnace walls can be damaged. This result is brought about by the presence of the water walls 35 and the lower tubes 23 of the boiler, which are exposed to the radiant heat. The high temperature is confined to the auxiliary furnace chamber 45, which is comparatively small. Moreover this auxiliary furnace chamber can be designed with a large proportion of water cooled surface since the combustion rate therein will ordinarily be both high and constant and can be predetermined. The design Will not be hampered by considerations of flexibility. The water wall tubes 70 are entirely separate from the tubes 35, and can be independently replaced in case of failure.

A large part of the fly ash which is carried out of the main furnace by the gases will be deposited in the pocket 78 and may be returned through the pipe 79 to the auxiliary furnace chamber. Here the combustible matter will be consumed and its heat energy recovered, while the residue will be melted and mixed with the liquid slag. This increases the emciency of the furnace and reduces g5@ claim as new and desire to secure by Letters Patent is:

l. A furnace comprising walls forming a main l furnace chamber arranged for burning pulverized fuel in suspension, means for introducing pulverized fuel and air into said chamber, walls forming an auxiliary furnace chamber in communication with the main chamber, said auxiliary chamber being arranged to receive incombustible matter deposited from the main chamber, means to maintain the auxiliary furnace chamber at a temperature higher than the fusion temperature of said incombustible matter, means to collect solid material which has been carried out of the main furnace chamber by the furnace gases, and means to deliver this material to the auxiliary furnace chamber for combustion and fusion therein.

2. A furnace comprising walls forming a primary combustion zone and a supplementalcombustion zone, means to introduce pulverized fuel and air into the primary zone for combustion in suspension, the supplemental zone being located beneath the primary zone in position to receive the incombustible residues deposited from the pulverized fuel flame, water cooled tubes to absorb radiant heat in the primary zone and hold the temperature normally beneath the fusion point of the residues, means to introduce fuel and air into the supplemental zone for combustion in suspension therein at such a rate as to maintain the temperature in the supplemental zone above the fusion temperature of said incombustible residues, irrespective of the combustion rate in the primary zone, and means to withdraw said residues from the furnace in liquid form.

3. A furnace comprising walls forming a main furnace chamber arranged for burning pulverized fuel in suspension therein, means for introducing pulverized fuel and air into said chamber, said chamber having a sloping bottom providing a restricted opening for the discharge of incombustible residues deposited from the fiame, water cooled tubes arranged to absorb radiant heat from the chamber and prevent the temperature in the chamber from exceeding under normal operating conditions the fusion temperature of said incombustible residues, walls forming a comparatively small auxiliary furnace chamber in position to receive the residues discharged through said opening, means to introduce fuel and air into the auxiliary furnace chamber for combustion in suspension therein at a rate sufficient to maintain the temperature in the auxiliary furnace chamber above the fusion temperature of said residues, the gases from the auxiliary furnace chamber passing into the main furnace chamber, and means to withdraw the residues from the auxiliary furnace chamber in the form of a liquid slag.

4. A furnace comprising a front wall, a rear wall, and two side walls forming a main furnace chamber arranged for burning pulverized fuel in suspension therein, means for introducing pulverized fuel and air into said chamber, said side walls having downwardly and inwardly sloping extensions which provide a narrow opening beneath the center of the chamber, water cooled metallic tubes located adjacent to the inner surfaces of the side walls and the sloping extensions to absorb radiant heat from the name, walls forming a comparatively small auxiliary'furnace chamber located directly beneath said narrow -opening in position to receive incombustible fuel residues passing downwardly therethrough under the influence of gravity, means to introduce fuel and air into the auxiliary furnace chamber for combustion in suspension at a sufficiently high rate to maintain the temperature therein above the fusion temperature of the incombustible residues and reduce said residues to a liquid slag, and means to withdraw the slag from the auxiliary furnace chamber in liquid form.

5. The method of operating a furnace comprising the steps of introducing pulverized fuel and air into the upper portion of the furnace for combustion in suspension, varying the rate of introduction in accordance with the demand on the furnace, collecting the incombustible res-v idues from the fuel in the bottom of the furnace, introducing additional fuel and air into the lower portion of the furnace for combustion in suspension at a substantially constant rate sufficient to melt said incombustible residues by a reverberatory action irrespective of the combustion rate of said pulverized fuel, and withdrawing said residues from the furnace in liquid form.

6. The method of operating a furnace having a main furnace chamber and an ash pit therebeneath comprising the steps of introducing pulverized fuel and air into the main furnace chamber for combustion in suspension, varying the rate of introduction in accordance with the demand for heat, collecting the incombustible residues deposited from the flame in the ash pit, burning additional pulverized fuel in suspension in the ash pit at a substantially constant rate sufficient to maintain the temperature in the ash pit above the fusion temperature of the incombustible residues, removing said residues from the ash pit in liquid form, and causing the gases from the ash pit to pass through the main furnace chamber.

'7. The method of operating a furnace having a main furnace chamber and an ash pit therebeneath comprising the steps of introducing pulverized fuel and air into the main furnace chamber for combustion in suspension, maintaining the temperature within the main furnace chamber below the fusion temperature of the incombustible residues deposited from the flame, collecting the said incombustible residues in the ash pit, burning additional fuel in suspension in the ash pit, maintaining the temperature in the ash pit .above the fusion temperature of the incombustible residues, and removing said residues from the ash pit in liquid form.

8. The method of operating a furnace having a main furnace chamber and an ash pit therebeneath comprising the steps of introducing pulverized fuel and air into the main furnace chamber for combustion in suspension, collecting the incombustible residues deposited from the ame in the ash pit, collecting solid material which has been carried out of the main furnace by the furnace gases, delivering this solid material to the ash pit, burning sufficient additional fuel in suspension in the ash pit to provide a temperature therein above the fusion temperature of the incombustible residues, and removing said residues from the ash pit in liquid form.

9. The method of operating a furnace having a main furnace chamber and an ash pit therebeneath comprising the steps of introducing pulverized fuel and air into the main furnace chamber for combustion in suspension, absorbing meme? introduction in accordance with the demand on the furnace, collecting the incombustible residues from the fuel in the bottom of the furnace, introducing additional fuel and air into the lower portion of the furnace for combustion in suspension at a rate scient to melt said incombustible residues by a reverberatory action irrespective of the combustion rate of said pulverized `fuel, and withdrawing said residues from the furnace in liquid form. 

